In recent years, ultra-thin and compact power storage devices have been actively developed for use in personal computers, mobile terminal devices such as cell phones, video cameras, satellites and vehicles. As a packaging material for such power storage devices, laminate packaging materials formed of a multi-layered film (for example, a configuration made up of a substrate layer/first adhesive layer/metal foil layer/second adhesive layer/thermal bonding resin layer) have been attracting attention. Unlike metal cans that have been used as a container, the laminate packaging materials formed of a multi-layered film are superior to the metal cans in light weight, high heat radiation and freedom of design.
The power storage device is produced, for example, by forming a recess by cold molding on part of the packaging material for the power storage device formed of a multi-layered film, accommodating a positive electrode, a separator, a negative electrode, an electrolyte solution, and the like in the recess, folding over the remaining portion of the packaging material to cover the recess, and sealing the edges of the packaging material by thermal bonding. In recent years, for the purpose of efficiently accommodating more contents to increase the energy density, power storage devices having recesses on both sides of the packaging material to be bonded together have been produced. However, as the recess depth increases, pinholes and breakage may often occur during a molding process by a mold at an edge or corner of the recess, which is a portion having a high degree of stretching.
In order to solve the above problem, an attempt has been made to reduce the occurrence of pinholes and breakage by using a film of biaxially stretched polyamide or the like as a substrate layer of a packaging material for a power storage device to protect a metal foil (see for example PTL 1).